This meeting addresses a fundamental shift in our understanding of chromosomal structures and their relation to gene activity and other chromosomal functions. These changes affect many areas of cell and developmental biology. Recent major discoveries have shown that transcription involves active participation of chromatin components and structure. Enzymes and other factors that alter chromatin structure and that chemically modify histones have been identified and shown to play a major role as co-factors that can activate or repress transcription at specific genes. Additional levels of chromatin packaging modification are involved in the formation of heterochromatin at centromeres and telomeres and in X-chromosome inactivation and imprinting. Many of the factors involved in these functions and the role of replication in their establishment are also part of the subject of this meeting. Finally, the larger scale organization of chromosomal structures and sister chromatid interactions are discussed. Speakers have been chosen to address a very wide range of topics, because it is clear that in all these fields common mechanisms related to chromatin structure are being uncovered. A large number of important new publications related to these problems are appearing each month, and it is difficult for anyone to maintain a coherent view of the field without meetings at frequent intervals. This is particularly true of graduate students and postdoctoral fellows studying problems of transcription, replication and DNA repair, who must integrate these results into their own work, but it is also true that more senior investigators specializing in these areas must now keep track of the coupling between them that arises through common dependence on chromatin structural changes.